The present invention relates to a lasting insole for a welted shoe, to a method of manufacturing a lasting insole and to an apparatus for manufacturing a lasting insole.
To manufacture a welted shoe, the margin of a lasted upper is stitched to a rib of a lasting insole, a welt is also stitched to the rib and then an outsole is fastened to the welt. A midsole may also be fastened to the welt. A welted shoe has several advantages, including good flexibility.
To manufacture a conventional or "Goodyear" lasting insole, a support layer is cut from leather, the support layer is slashed along its periphery, the slashed periphery is turned out at a 90.degree. angle to form a rib and then a textile strip is tacked to the rib and to the support layer to reinforce the rib.
The conventional method of manufacturing a lasting insole is disadvantageous for at least two reasons. First, the method requires an undue amount of leather. This is because the leather support layer must be quite thick so that the rib (which is slashed from the support layer) can be adequately thick. Both of the slashed parts must be adequately thick to keep sufficient strength. Second, the method is unduly complicated.
In an improved method of manufacturing a lasting insole, a support layer is cut from leather (natural leather, artificial leather or another appropriate material), a rib is formed from elastomeric material (such as rubber, polyurethane or polyvinylchloride) and then the elastomeric rib is bonded to the support layer by an adhesive connecting film which is applied to the support layer. To achieve the needed bond strength, the connecting film is formed of a material which bonds both to the material of the support layer and the material of the rib. This method is less complicated than the conventional method. Further, the improved method does not require as much leather as is required by the conventional method.
However, creating the appropriate bond between the material of the support layer and the elastomeric rib is difficult. The elastomeric rib and the support layer tend to separate from each other during use. The bond between the rib and the support layer deteriorates because stresses encountered during use are maximized at the interface between the rib and the support layer, i.e., where the edges of the rib meet the surface of the support layer. That is, when wearing the welted shoes, the rib is very stressed and thus the bond between the rib and the support layer is simultaneously stressed. The critical place of the bond is in the interfacing lines between the margins of the rib and the surface of the support layer.
The improved lasting insole is manufactured within an injection molding apparatus. In operation, the support layer is split or ground to a nominal thickness and then placed within a die part of the apparatus. In particular, the support layer is placed within a cavity which has a depth which is less than the nominal depth of the support layer. A punch part is then pressed against the die part.
As the punch part is pressed against the die part, a shaping cavity defined within the punch part is supposed to be sealed against the support layer to form the rib. But the shaping cavity is often not properly sealed to the support layer because the thickness of the leather support layer varies with changes in humidity (natural and artificial leathers are hygroscopic materials) and/or the support layer is not ground to the proper nominal thickness. As a result, some of the injected material which is supposed to form the rib escapes out of the shaping cavity. The injection molding apparatus operates with accurately measured batches. Therefore, when some of the injected material escapes, the resulting rib is incomplete and unsatisfactory.
Sherbrook, British Patent Specification No. 1,078,082, discloses a lasting insole with a rib which is formed of several layers of flexible material. Sherbrook's insole is unduly complicated.
Habrovansky, Czechoslovakian Author's Certificate No. 251,741, discloses a lasting insole which is formed entirely from plastic material. This material does not have the permeability of natural or artificial leather, which is important for hygienic reasons. Habrovansky's insole is not sufficiently permeable, even though holes are formed through the insole in an attempt to increase permeability.
Sevela, Czechoslovakian Author's Certificate No. 254,361, discloses a lasting insole which, like the improved insole described above, includes a leather support layer and an injection molded rib. Sevela's rib has integrally formed rivets which are supposed to be anchored in holes through the support layer. But the rivets often do not extend through the holes as desired and, as in the improved lasting insole, stress encountered during use is maximized along the interface between the rib and the support layer. Thus, Sevela s insole is not sufficiently durable. Further, the manufacture of Sevela's insole is unduly complicated.